The invention relates to a platen clamping unit for an injection molding machine of a type set forth in the preamble of claim 1.
Such platen clamping units are generally known in plastics injection molding machines or in injection molding machines for thixotropic metals. The moving mold mounting plate and the fixed mold mounting plate, each carry a half-mold. Firstly, the platen clamping unit has the function to move the mold mounting plates relative to one another and to thereby open or close the mold. A further function resides in the application of a high clamping force to ensure that the mold remains closed even at very high injection pressures and is prevented from moving apart. Although, it is conceivable to unite both functions in one drive unit, this however results in different demands of the construction. Movement of the plates can be implemented at relatively small force while a fairly large stroke is, however, needed; clamping requires a relatively small stroke at a high clamping force. As a consequence, it is difficult to dimension such a single drive to meet both functions.
Separate units were therefore proposed, for example, in EP 0 544 903 B1, in which a single drive implements the movement and electromagnets ensure the clamping force; a combination of ball screw and toggle lever is shown in EP 0 383 935 A1; an arrangement of toothed racks and eccentric is known from DE 195 44 329 A1.
The known techniques for the clamping force unit require cumbersome and maintenance-intense mechanisms and very high clamping forces can only be generated to a limited degree.
The so-called 2-platen clamping units afford a particular compact configuration. The conventional configuration (3-platen structure) involves the provision of a support plate in addition to the fixed mold mounting plate, whereby the moving mold mounting plate is arranged between the fixed mold mounting plate and the support plate and mechanically supported upon the support plate. When the 2-platen structure is involved, the moving mold mounting plate is supported only upon the fixed mold mounting plate. Although this constitutes a relatively compact configuration, there are, however, problems when a clamping force unit should be attached.
A platen clamping unit according to the preamble of claim 1 is known from EP 0 381 107 B1. In this conventional platen clamping units, the moving mold mounting plate is provided in the area of the ball screw with an additional hydraulic drive for applying a very high clamping force. However, this structure is mechanically very complicated and incurs high costs to implement and may introduce additional play or tolerances into the system, so that the parallelism of the mold mounting plates, required for implementing a precise operation of the platen clamping unit, cannot always be ensured.